1. Field of the Invention
This invention relates to the centrifugal casting of iron pipe and more particularly to a process for centrifugally casting iron pipe in a metal mold which enables use of a lighter weight mold and extends the useful life of the mold.
2. Description of the Prior Art
The production of cast iron and ductile iron pipe by the deLavaud system incorporating permanent or semipermanent metal molds and utilizing a centrifugal casting procedure is well known in the art as illustrated, for example, in U.S. Pat. No. 1,949.433. Such a system employs a pouring ladle for receiving the molten iron and accurately pouring a predetermined amount of the molten iron within a predetermined length of time into a fixed trough positioned on an incline to carry the molten metal to the mold contained within and rapidly rotated by the casting machine. The casting machine is reciprocated along a track in a rectilinear motion at a predetermined rate whereby the fixed trough is inserted into and withdrawn from the open end of the mold in the casting machine. Molten metal poured from the ladle flows from the trough and is deposited at a uniform rate into the mold during the withdrawal movement.
The typical deLavaud casting machine includes a water jacket surrounding the metal mold, with the mold being either submerged in a water bath in the jacket or with spray means provided within the jacket applying cooling water in high pressure streams onto the outer surface of the rotating mold. The metal mold is thus continuously cooled, either by the water bath or water spray, throughout each successive casting cycle. Each mold is formed from a high quality steel forging or casting which is precision machined to dimensions required for casting pipe of a standard size. Relatively close tolerances with respect to roundness and straightness are required because of the high speed of rotation necessary to centrifugally cast the molten metal. Also, the mold has wall thickness and strength requirements to prevent warpage and distortion due to the thermal stresses induced during the casting process. The mold thickness used in the commercial production of iron pipe is generally based upon achieving dimensional stability throughout an acceptable life span for the mold.
Mold thicknesses used in centrifugal casting of iron pipe vary with the diameter of the pipe being produced and generally are represented as a mold diameter to mold wall thickness ratio. For example, for casting pipe having a diameter of 4", this ratio may be about 6, with the ratio increasing to about 16 for 24" pipe whereas, for molds greater than 24" in diameter, the mold diameter to thickness ratio will generally remain within the range of about 18 to 22. Practical experience has shown that, for these ratios, molds accurately produced from a high strength steel will retain the required dimensional stability over an economically acceptable life span.
During the casting operation, the inside surface of the metal mold is rapidly heated while the outside surface is maintained at a relatively low temperature by the cooling water. For example, the inside surface of the mold may be heated to about 1300.degree. F. within about one second after contact with the molten iron while the outside mold surface, being continuously cooled, will normally never exceed about 400.degree. F., depending upon mold size and operating conditions. This severe temperature gradient through the mold wall results in high thermal stresses in the mold, with the inner portion of the mold wall being in a state of compression and the outer portion in tension. The compressive stresses on the inner portion of the mold are so great as to cause compressive yielding with plastic compressive strain during this high temperature phase of the molding cycle. A few seconds later, as the thermal gradient across the mold wall thickness reaches its maximum, the tensile stresses at the outer surface will also reach their maximum. This maximum tensile stress at the outer surface will occur when the difference between the mean mold temperature and the outer surface temperature are at a maximum.
As the cast pipe solidifies, an air gap will form between the outer surface of the solidifying casting and the inner surface of the mold, with a consequent substantial reduction in the thermal load, or heat flow, from the casting to the mold. When separation between the mold and casting occurs, the temperature of the inner surface of the mold begins to drop and compressive stresses are relaxed. Continued cooling results in the inner surface portion of the mold going into tension due to the previous compressive yielding and, with continued cooling, the tensile stresses build until they exceed the yield strength and plastic tensile strain results. Thus, during each casting cycle, the portion of the mold adjacent the inner surface undergoes compressive yielding followed by tensile yielding which eventually leads to failure by the formation of cracks on the inner surface. This type of failure is known as low cycle thermal fatigue, and continued use causes the inner surface cracks to grow until it becomes difficult to extract the casting from the mold, or the cracks may become so severe as to mar the surface of the casting to the extent that it is unacceptable from an appearance standpoint.
Molds may also fail by tensile yielding of the outer portion of the mold wall which produces mold warpage or out of roundness. This can occur with or without the presence of cracks on the inner surface, and is usually indicative of a mold formed from a material having relatively low yield strength or a mold having an improper wall thickness for the operating conditions. Operating conditions may be influenced by various factors such as casting rate, molten metal temperature, cooling rate and the like. Molds which fail as a result of outside tensile yielding generally cannot be economically repaired and must be scrapped.
In order to increase mold life by reducing thermal fatigue cracks on the inner surface, the range of compressive and tensile strain (strain range) must be decreased. The relationship between the strain range and the number of cycles to failure is a log-log relationship, with the consequence that a small decrease in the strain range may result in a substantial improvement in mold life. The strain range may be reduced by a wet spray process involving the application of a thin refractory wash sprayed onto the inner surface to reduce the thermal load by putting an insulating layer between the molten iron and the mold surface. While this technique extends mold life, it is not entirely satisfactory from a commercial standpoint in that it substantially reduces the production rate due to the extra time required to apply and dry the refractory lining on the inner surface of the mold before each casting cycle.
It is also possible to reduce the thermal load by reducing the pouring temperature of the molten iron but there are obvious practical limitations on the amount this temperature can be reduced and still successfully pour a sound casting. Casting of pipe having standard sizes and wall thicknesses thus essentially fix the thermal load.
Another technique to reduce the strain range is to decrease the mold wall thickness since a thinner mold has a lower strain range. As the mold wall thickness is decreased, the tensile stress on the outside surface increases, however, and plastic yielding of the outer surface fibers may occur, again placing a practical limitation on this method. In practice, molds having a diameter to wall thickness ratio within the ranges discussed above have been found to provide a practical compromise, while molds having a diameter to wall thickness ratio greater than about 25 may warp beyond use in less than 100 casting cycles using conventional casting techniques.
It is, therefore, an object of the present invention to provide an improved process for the centrifugal casting of cast iron and ductile iron pipe.
It is a further object of the invention to provide such a method which will greatly extend the useful life of the metal mold in which the pipe is cast.
It is a further object to provide such a process which will enable use of metal mold having thinner walls than previously used in commercial pipe molding operations.
Another object is to provide such a process in which the thermal stress across the mold wall thickness is reduced to thereby reduce the strain range in the mold wall during each casting cycle.